1. Field of the Invention
The present invention relates to an image reading apparatus and image data processing method. Particularly, the present invention relates to an image reading apparatus and image data processing method for correcting an abnormal image caused by a smear when reading an image by optically scanning the image original.
2. Description of the Related Art
Some conventional image reading apparatuses use a contact image sensor (to be simply referred to as a CIS) as an image sensor for scanning an original. An apparatus of this type sequentially switches and uses three LEDs for emitting light beams of R, G, and B color components to irradiate the original surface with light. Thus, image data are read out line-sequentially in order of the color components of one line.
FIG. 8 is a timing chart showing LED turn-on and readout in image reading.
As shown in FIG. 8, one line (cycle of a signal Lsync) is divided into three periods (cycle of a signal Hsync). LEDs are turned on in order of a red LED LED_R, green LED LED_G, and blue LED LED_B, storing charges. The stored charges are transferred outside the LED during the period of the signal Hsync in the next cycle. For example, during period “A”, exposure and charge storage by the red LED are performed. At the same time, charges obtained by exposure and charge storage by the blue LED for a preceding line are transferred outside.
FIG. 9 is a view showing the schematic arrangement of the image sensor.
The LED irradiates an image original with light. When the light reflected by the image original enters the array of a photodiode 201, charges are stored to generate an image signal. The signal charges stored in the array of the photodiode 201 are sent to a vertical transfer register 202, and held for a period until they are horizontally transferred. At the transfer timing, the signal charges are sent to a horizontal transfer register 203. The signal charges are then transferred to an output circuit (not shown) via the horizontal transfer register 203.
In this arrangement, portions except for the photodiode 201 are shielded from light by an aluminum light shield (not shown) to prevent generation of unwanted charges. However, the light shielding is sometimes insufficient due to layout restrictions of the apparatus and device, or the like. For example, when an aluminum wire for transferring an electrical signal is used even for light shielding and a wire different in potential is arranged, this wire cannot be connected to the signal line. An aluminum wire slit is generated between non-equipotential portions, degrading the light shielding ability compared to the remaining portion. In this case, unwanted light enters from the slit, generating unwanted charges. The unwanted charges are added to a normal signal, outputting a signal higher in strength than the normal signal (signal indicating a brighter state). This phenomenon is called a smear.
A fundamental countermeasure against the smear is to prevent incidence of unwanted light. For example, if a smear occurs due to unwanted light entering a charge transfer portion such as a vertical transfer register or horizontal transfer register, enhancing a light shield above the charge transfer portion so as to prevent incidence of unwanted light can be a fundamental countermeasure. However, the wafer area of the circuit board in recent apparatuses is decreasing for cost reduction, and it becomes difficult to achieve a good aluminum light shielding effect. On the other hand, the degree of aluminum light shielding effect depends on the manufacturing precision of the aluminum light shielding plate, and less varies. This is the structural issue of the aluminum light shielding plate, so a smear occurs at the same location if the incident light quantity is the same.
Conventionally, countermeasures to correct degradation of the image quality caused by the smear have been proposed. For example, in Japanese Patent Laid-Open No. 2007-201553, after charges generated by a plurality of light reception elements effective for image capturing are transferred to a charge transfer portion, correction data is generated based on data obtained from an output signal in an additional transfer operation executed in addition to a transfer operation for the charges received by the charge transfer portion.
However, the conventional technique requires an additional charge transfer operation, prolonging the time taken for image reading. In the conventional technique, since no smear occurrence position is known in advance, it is necessary to locate where the smear occurs. If the detection fails, correction processing becomes less effective. Further in the conventional technique, the amount of exposure by the light emitting element is not always constant. The occurrence amount of every smear varies, impairing the correction effect.
The degree of influence of output level variations caused by the smear changes depending on the irradiation light quantity of the LED. Thus, uniformly subtracting the light quantity does not lead to correction. For example, in a line-sequential reading method of reading out signals by sequentially switching and turning on red, green, and blue LEDs, the brightness levels of the R, G, and B color components change depending on the color balance of an original. The brightness level is a reflected light quantity level obtained from reflected light of light which irradiates an original. When the reflected light quantity level changes, the smear amount also changes, and uniform correction does not work.
In the first place, the smear occurs when charge transfer and exposure are executed simultaneously. Occurrence of the smear can, therefore, be prevented by performing exposure and charge transfer in different periods. However, performing exposure and charge transfer in different periods results in prolonging the image reading time and decreases the performance.
The time necessary for charge transfer may be shortened by quickly executing charge transfer. If the time necessary for charge transfer can be shortened, exposure is performed in the remaining time, enabling execution of charge transfer and exposure in different periods. However, implementation of fast charge transfer requires a high-speed clock, increasing unwanted radiation and noise.
Charge transfer and exposure may be performed in different periods by increasing the irradiation light quantity from the light source to shorten the exposure time. However, introduction of a high-output light source has problems such as high apparatus cost and light quantity variations under the influence of heat generated by the high-output light source.